Figure 36-17 Time and volume capnographs. A, Expired PCO₂ versus time (i.e., standard time capnogram). The waveform is conventionally subdivided into phases. During phase I, exhaled gas from the large airways has a PCO₂ = 0. Phase II is the transition between airway and alveolar gas. Phase III (i.e., alveolar plateau) is normally flat, but in the presence of V̇A/ mismatching, it has a positive slope. The down slope of the capnogram at the onset of inspiration is usually referred to as phase IV, but there is sometimes a terminal increase in the slope associated with the onset of airway closure (dashed line labeled IV'). This corresponds to the terminal upstroke seen in single inert gas washout curves, referred to in that setting as phase IV.^[322] The PCO₂ value at the end of exhalation is referred to as the end-tidal PCO₂ (PETCO₂ ). Also shown are the exhaled gas flow rate and volume. B, Volume capnogram. In this form of the capnogram, exhaled PCO₂ is plotted against exhaled volume. Mixed expired PCO₂ can be measured for each breath as the area under the capnogram. Total physiologic dead space (VDS PHYS) can there fore be measured using arterial PCO₂ and Equation 12 (Bohr equation, assuming PACO₂ = PACO₂ ). Line AC is drawn tangent to the terminal portion of the alveolar plateau. Vertical line BE is constructed such that the two shaded areas (EDG and BCG) are equal in area. FE therefore represents anatomic dead space (VDS ANAT), ^[323] which includes the volume in the trachea and large airways and any volume within a breathing circuit in which exhaled gas is rebreathed, such as the endotracheal tube, passive humidification device, or Y-piece. Alveolar dead space (VDS ALV)^[323] can therefore be calculated as the difference between VDS PHYS and VDS ANAT.^[324] Because the area of trapezoid BCDE is equal to the volume of CO₂ exhaled per breath, the mean (or average) alveolar PCO₂ is the value at the midpoint of segment BC (point P).^[324]